The role of single cardiac muscle fibers in the origin of cardiac arrhythmias is to be studied. The transmembrane potentials of single fibers in isolated tissue preparations obtained from the dog heart will be used. The type of fibers under study are atrial plateau fibers, Purkinje fibers and ventricular muscle fibers. The primary objective is to study the conditions under which these fibers develop what are termed "slow responses." Slow responses are action potentials which result when rapid depolarization processes mediated by sodium ions are inhibited. It is believed that the occurrence of slow responses may be involved in the origin of certain types of cardiac arrhythmias, notably those associated with occlusion of a coronary artery. It is of interest to determine if atrial plateau fibers display the slow response. Further study of slow responses in Purkinje fibers also is planned. Particular attention is being given to the characteristics of phase 4 depolarization which occurs simultaneously with the development of slow responses. The data will be interpreted with the idea of contributing to the understanding of the production of cardiac arrhythmias, especially those which arise secondarily to cardiac ischemia.